Currently, AFCS (Automated Flight Control Systems) in business class jets operate with two fundamental climb functions, Vertical Speed and Flight Level Change (FLCH).
Vertical Speed Mode is commonly utilized to provide a constant feet per minute rate of climb (vertical speed) based on a manual selection by the pilot. While this is an effective method to climb while the autopilot is engaged, it requires constant input from the pilot to adjust climb rates to ensure safe attitudes (the relation of an aircraft to its surroundings such as air flow and gravity, particularly the orientation of the aircraft with respect to the horizon) as the aircraft climbs through higher altitudes. If the Vertical Speed selection is left unattended, the airplane will attempt to maintain the climb rate through attitude adjustment with no regards to diminishing speed. Subsequently, it is quite possible to achieve a high altitude stall if the pilot is distracted and forgets to adjust the Vertical Speed according to aircraft altitude and performance.
FLCH Mode is frequently taught by training organizations as the primary method to climb via the AFCS because it maintains a constant air speed during the climb. This method virtually eliminates the possibility of stalling the aircraft during the climb through the flight levels. Unfortunately, the FLCH Mode cannot accommodate the constantly changing air data parameters affecting the aircraft speed through the air. Consequently, during a FLCH selected climb, the aircraft tends to oscillate up and down on the pitch axis (the orientation of the aircraft's longitudinal axis, whether the aircraft's nose is pointing up, down, or level with the horizon) in an effort to maintain a constant air speed through constantly changing temperatures at altitude. The result is an extremely erratic climb and a sometimes uncomfortable ride for the crew and passengers.
Consequently, it would be desirable to provide an automatic flight control system vertical control function which does not suffer from the drawbacks of Vertical Speed Mode and FLCH Mode.